Software will be developed for IBM PC and compatible running MS-DOS or PC-DOS. The proposed software (FitSolver) will allow scientists and engineers to quickly and easily solve a wide range of nonlinear equations and to estimate parameters for these equations. The nonlinear equations include algebraic equations, delay equations, ordinary and partial differential equations. This software will provide users with the capabilities far beyond those available from other similar products. These capabilities include: the choice of a variety of numerical techniques for nonlinear algebraic equations (bisection, Newton's, steepest decent, Powell's, conjugate gradient); ordinary differential equations (Bulirsh-Stoer, Gear, and ISODE); partial differential equations (finite difference, finite element, method of line, moving finite element); and parameter estimation (least-square, fixed-point projection, space marching). This software will also provide easy-to-use graphical user interface that will allow scientists and engineers solve their problems arising from physical, chemical, and biological systems in an easy and simple way. This software will be a fast, easy-to-use, and powerful model development and data analysis tool for scientists, engineers, and college students. It can also be very useful for educational purposes.